Annabel Croft believes Great Britain will need to win the doubles rubber if they are to beat the USA in the Davis Cup this weekend - but says doing that is "as big a task as it gets".

The former British number one, who will be presenting British Eurosport's coverage of the World Group first-round tie, feels the USA have the edge due to the fact they will field the world's best doubles team, Bob and Mike Bryan, in Saturday's doubles clash in San Diego.

The singles matches, which take place on Friday and Sunday, look likely to be contested by Andy Murray and teenager Kyle Edmund for Britain and John Isner and Sam Querrey for the hosts, Isner being set to play despite admitting he's still not 100 per cent recovered from the ankle injury which forced him out of the Australian Open.

"It's probably going to come down to the doubles," said Croft.

"Andy Murray is now the world number six but somebody of his calibre should win two (singles) matches. The quality of America is better than our number two so I'm assuming he won't win two matches.

"That will leave it down to the Bryan brothers versus probably Murray and Colin Fleming."

With Murray looking set to team up with doubles specialist and fellow Scot Fleming, Britain are likely to field a pairing who have only played together on rare occasions. The Bryan twins, however, have played together all their life - and that is likely to make all the difference says Croft.

"The top singles players on a doubles court are still top players, it's just they don't play that much doubles. They are still worth having on the court.

"But anyone up against the Bryan brothers, that is a huge task. That's not to say they're not beatable but they are probably the greatest doubles team ever, playing in their home country, on their home court (they were born in California).

"That's as big a task as it gets.

"They did lose in Australia so they might be coming in not quite as confident but over the best-of-five sets and with the record they have... They've won every title going, 95 doubles titles is phenomenal."

As for the singles, Murray will be under "enormous pressure", according to Croft and two wins should not be taken for granted, particularly with the US having chosen Murray's least-favourite surface, clay, for the tie.

"Isner has had some great results on clay and Andy doesn't like playing on it," she added. "He's also going to be under enormous pressure as Britain's hopes rest with him.

"Isner is never easy and he beat Roger Federer on clay a few years ago. Sam Querrey has dropped in the rankings but has beaten Andy Murray in the past on hardcourts. He's talked that up and it will be good for him to take that memory into the match."

Murray has already admitted his frustration at having to move from the hardcourts of the Australian Open, where he lost to the quarter-finals, to the clay at Petco Park, home of San Diego's baseball team, the Padres.

The surface switch will not help his back, which was operated on in September, as Croft explained.

She said: "He trained on in Melbourne before heading to the US at the weekend so he's had almost a week to get used to the change of surface.

"He'll have played quite a bit on the clay for the last week but it's never going to make up for a lack of matches on it, so it's tough.

"It's the movement on the clay which is much more difficult and after his back surgery last year it's just extra stress on the back to be able to move on the clay."

However, while some viewed Murray's comments about the surface switch negatively, Croft feels he will very much be up for the tie.

"He's an unbelievable competitor, one of the most stubborn players I've seen. I think he's enjoying Davis Cup more and more and now we're back in the World Group he will be fussed about it."

The tie gets under way at 1900 GMT on Friday and lasts until Sunday. The winners will progress to April's quarter-finals with the losers facing a World Group relegation play-off in September.

British Eurosport will broadcast live coverage of Great Britain's Davis Cup World Group first-round tie against USA from January 31 to February 2.